Problem: Simplify the following expression: $r = \dfrac{3x}{8} + \dfrac{3x}{4}$
Explanation: In order to add expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $8$ and $4$ $\lcm(8, 4) = 8$ $ r = \dfrac{1}{1} \cdot \dfrac{3x}{8} + \dfrac{2}{2} \cdot \dfrac{3x}{4} $ $r = \dfrac{3x}{8} + \dfrac{6x}{8}$ $r = \dfrac{3x +6x}{8}$ $r = \dfrac{9x}{8}$